This two-volume set LNCS 15516-15517 contains revised selected papers from the 31st International Conference on Selected Areas in Cryptography, SAC 2024, held in Montreal, QC, Canada, in August 2024. The 25 full papers presented in these proceedings were carefully reviewed and selected from 95 submissions. The papers are organized in the following topical sections: Part I: Privacy-preserving cryptography; post-quantum cryptography; attacks on public-key cryptography; and identity-based encryption. Part II: Authenticated encryption; symmetric design strategies; cryptanalysis of arithmetization-oriented primitives; symmetric primitive design; and implementations and side-channel analysis.